The inventive subject matter relates to motor drive apparatus and methods and, more particularly, to variable-speed drive apparatus and methods.
Acceleration of an AC motor can cause large inrush currents and produce uncontrolled torque that can generate damaging mechanical shock. Accordingly, large motors are often started using a soft starter or a variable frequency drive (VFD). A soft starter reduces inrush current by limiting the voltage applied to the motor until the motor is up to speed. A VFD can provide even greater control by varying the output voltage frequency to allow the motor to gradually be brought up to speed at rated torque.
A typical application of a VFD for motor starting and stopping is illustrated in FIG. 1. A VFD 20 has an input coupled to an AC source 10 and an output coupled to a load, such as a motor 30. A bypass switch 40 is configured to directly couple the AC source to the motor 30, bypassing the VFD 20. To start the motor 30, the VFD 20 is used to accelerate the motor 30 up to a desired frequency while the bypass switch 40 is open. When the desired frequency is reached, the VFD 20 is disconnected from the motor 30 and the bypass switch 40 is closed. To stop the motor 30, the VFD 20 output is brought to the desired frequency and connected to the motor 30 after the bypass switch 40 is opened.